According to current practice, fuel-injected internal combustion engines are supplied with fuel by means of electrically-driven fuel pump units disposed within the fuel tanks of these engines and immersed in fuel. Electric motors forming parts of these units normally utilize conventional copper commutators. However, wear of these copper commutators is surprisingly heavy. This wear, it is thought, arises from the combined effects of abrasion by the carbon brushes engaging the commutator and electrolytic erosion resulting from the small but common water content of commercially available fuels. These two influences appear to operate synergistically. It therefore seems desirable to replace copper commutators with non-metallic commutators which are not subject to the same degree of electrolytic erosion when immersed in fuel contaminated with water.
One known planar carbon segment commutator, disclosed in the specification of German utility model G 89 07 045.3, in the joint names of Deutsche Carbone AG and Robert Bosch GmbH, comprises an insulating support member having a circular face, copper contact members mounted on the circular face of the support member, and a carbon layer provided outside each contact member by injection molding a mixture of carbon powder and carrier material.
An outer rim on each contact member supports the carbon layer mounted on the contact member against centrifugal force and, to support the carbon layers against axial displacement, each layer is formed integral with at least one axially extending anchor piece, during the injection molding process, after providing axially extending holes in the support member and/or the contact members for receiving the injection molded mixture. These anchor pieces need not be very strong to prevent axial removal of the carbon layers because there is little axial loading on the carbon layers urging them away from the contact members. The brushes engaging the commutator actually assist the anchor pieces by pressing the carbon layers against the contact members and the support member.
One known cylindrical carbon segment commutator comprises a base member of insulating material, such as liquid crystal polymer or phenolic resin, having an axially extending outer surface; a plurality of elongate axially-extending contact members; and a plurality of carbon segments respectively connected to the contact members and held in fixed relation to the base member.
In this known construction, the contact members comprise copper segments and the carbon segments have metal coated inner surfaces which are soldered to the copper segments constituting the contact members.
Construction of carbon segment commutators such as this involves the costly and time consuming steps of electroplating, or otherwise coating, the inner surfaces of the carbon segments with a highly conductive metal, such as copper, and then soldering the metal-plated surfaces of the carbon segments to the copper segments.